Pay attention to exports when determine which libraries to reload.

runtime/vm/unit_test.h

Index: runtime/vm/unit_test.h
diff --git a/runtime/vm/unit_test.h b/runtime/vm/unit_test.h
index 5a72dbb3d2cad903f3f7d4775da148806e6ba21f..8622a4f2eadfa979e6ac1339a744b10a4eac4384 100644
--- a/runtime/vm/unit_test.h
+++ b/runtime/vm/unit_test.h
@@ -314,9 +314,8 @@ class TestCase : TestCaseBase {
   // Helper function which reloads the current isolate using |script|.
   static Dart_Handle ReloadTestScript(const char* script);
 
-  // Changes the default importable test lib script.
-  static void SetImportableTestLibScript(const char* source);
-  static void RestoreImportableTestLibScript();
+  static void AddTestLib(const char* url, const char* source);
+  static const char* GetTestLib(const char* url);
 
  private:
   static Dart_Isolate CreateIsolate(const uint8_t* buffer, const char* name);